The present invention relates to a discharge lamp comprising a translucent ceramic tube dosed with a metal halide.
As this kind of a lamp, there is a high pressure discharge lamp as described in Japanese Patent Unexamined Publication No. Hei. 6-196131. As shown in FIG. 8, the arc tube of this lamp comprises a tube body 100 which is made of translucent ceramic such as polycrystalline alumina and is tapered at both ends thereof to form a narrow tube portion 101 at an end thereof, and an electrode lead 103 which is connected to an electrode 102 and inserted in and sealed to the narrow tube portion 101 with a sealing glass 104.
However, this structure has difficulty in providing the arc tube with a higher output. In order to provide a higher output, the diameter of the tube body 100 must be increased to prevent the temperature of the tube body 100 from rising to an abnormally high temperature. This makes the difference in diameter between the tube body 100 and the narrow tube portion 101 too great, making it difficult to produce this structure and causing cracks to occur more easily due to thermal impacts. On the contrary, when the diameter of the narrow tube portion 101 is increased, the gap between the narrow tube portion 101 and the electrode lead 103 increases, adding to the thickness of the sealing glass layer 104 with which they are sealed to each other. Such a sealing glass layer 104 can crack.
As shown in another structure (FIG. 9) described in the above-mentioned patent publication, there is an effective structure comprising a straight tube 105 of translucent ceramic, a ceramic terminal plate 106 fixed at both ends of the straight tube 105, a ceramic narrow tube 107 attached to the terminal plates 106, and an electrode lead 108 inserted in the narrow tube portion 107.
Although this arrangement makes it possible to provide a higher output and makes it easy to produce, a new problem was found that this arrangement gives a lowered luminous efficiency.
The reason for this problem can be thought as follows. In general, the arc tube is dosed with a halide in an amount far more than that required to be vaporized during lighting of lamp. Thus, extra halide left unvaporized is accumulated in a low temperature zone inside the arc tube. When this extra halide is accumulated in the arc tube, emitted light is absorbed by the halide thus accumulated, reducing the amount of light emitted by the arc tube and hence reducing the efficiency of lamp. When the arc tube is actually observed, it is recognized that the poorer the efficiency of the lamp, the more the amount of halide which has been deposited on the lower inner wall of the main tube is lit in a horizontal burning position. Since the structure shown in FIG. 9 comprises the straight tube 105 having the same diameter along the axial length, the temperature is low at the periphery of the both ends of the straight tube 105 (zone A indicated by the shadow). Excess halide left unvaporized resides in these zones to lower the luminous efficiency.
A first object of the invention is to improve the structure of the tube body, making it possible to provide a high output discharge lamp at a low cost. A second object of the invention is to further improve the luminous efficiency.
The first invention for solving the foregoing problems lies in a discharge lamp comprising an arc tube made of translucent ceramic dosed with a metal halide, and electrodes provided in the arc tube whereby discharge occurs across the electrodes, characterized in that the arc tube comprises a main tube body having integrally a large diameter portion, a tapered portion which is positioned at both ends of the large diameter portion and has a smaller diameter toward the forward end thereof and a small diameter portion which is continuously provided at the forward end of the tapered portion, a terminal plate of ceramic airtightly fitted in and fixed to the interior of the small diameter portion at the both ends of the main tube body, and a narrow tube of ceramic having the terminal plate airtightly inserted and fixed therein, the narrow tube having an electrically-introducing member connected to the electrodes inserted therein and airtightly sealed thereto with a sealing glass.
This arrangement can be produced more easily than the conventional structure having a narrow portion integrally formed at both ends of a ceramic tube body, making it possible to drastically reduce cost. Further, since the tapered portion is disposed at both sides of the large diameter portion of the main tube body so that the main tube body narrows at both ends thereof, a low temperature zone can be only with difficulty formed inside the arc tube during discharge, making it possible to prevent excess unvaporized halide from residing in the low temperature zone and causing the deterioration of luminous efficiency.
The second invention lies in the foregoing invention wherein supposing that the inner diameter of the large diameter portion of the main tube body is A and the inner diameter of the narrow tube is B, A and B satisfy the relationships:
B greater than 1.3 mm and 4.5xe2x89xa6A/Bxe2x89xa611.5.
In accordance with the present invention, the inner diameter of the narrow tube is not smaller than 1.3 mm, making it possible to use a large electrode and hence provide a discharge lamp with a great power consumption. Further, since supposing that the inner diameter of the large diameter portion is A and the inner diameter of the narrow tube is B, A and B satisfy the relationship 4.5xe2x89xa6A/Bxe2x89xa611.5, the resulting lamp efficiency is raised. When this numerical value A/B falls within a range of from not smaller than 6 to not greater than 10 (6xe2x89xa6A/Bxe2x89xa610), the resulting lamp efficiency is further raised.
Moreover, in accordance with the invention comprising a fitting member, which is made of ceramic sleeve or metal pipe and covers a part of the electrically-introducing member, fitted between the electrically-introducing member and the narrow tube, even if the inner diameter of the narrow tube is not smaller than 1.3 mm, the thickness of the sealing glass layer formed between the electrically-introducing member and the narrow tube can be reduced, making it possible to prevent the crack occurrence of the narrow tube during sealing and hence early leakage of air from the sealing glass layer due to heat cycle developed by switching of lamp on and off.